Water absorbent cores containing hydrophilic fiber, such as pulp, and a water absorbing resin particles are widely used conventionally so that sanitary/hygienic materials, such as disposable diapers, sanitary napkins, and incontinent pads, can absorb body fluids. The water absorbent core is used in sanitary/hygienic materials, such as disposable diapers, sanitary napkins, and incontinent pads, to absorb body fluids.
There are recent demands for these sanitary/hygienic materials to be reduced in thickness for better usability. Therefore water absorbent cores are manufactured with a decreasing ratio of hydrophilic fiber, which has a relatively low bulk density, and an increasing ratio of water absorbing resin particles, which exhibit excellent water absorption and a relatively high bulk density. The relative quantity of water absorbing resin particles used in the water absorbent core is hence increased, which in turn reduces the thickness of the sanitary/hygienic materials without compromising water absorbency and other physical properties.
The ratio of the hydrophilic fiber may be decreased, but not further below a minimum quantity required. For further reduction in thickness of the sanitary/hygienic materials, the physical properties of the water absorbing resin particles need to be improved. Examples of such physical properties of the water absorbing resin particles include centrifuge retention capacity, saline flow conductivity, absorbency against pressure, fixed height absorbency, mass median particle size, and liquid distribution velocity. These physical properties of the water absorbing resin particles need to be in predetermined ranges or excellent in actual use.
In, for example, a diaper which contains a sanitary/hygienic material with a high proportion of water absorbing resin particles, the water absorbing resin particles absorb water and changes into gel or a similar condition. That may lead to a phenomenon called gel blocking. The phenomenon reduces liquid diffusibility of the sanitary/hygienic material.
A method of adding inorganic particles to the water absorbing resin particles has been proposed to improve the physical properties of the water absorbing resin particles and address the gel blocking problem. According to the method, the inorganic particles are present between the water absorbing resin particles, thereby preventing the water absorbing resin particles from aggregating. The gel blocking problem is mitigated.
Patent document 1 discloses a water absorbing agent composition as water absorbing resin particles used with inorganic particles. The composition contains crosslinked, water-swelling resin powder and hydrophobic superfine particulate silica. The composition is intended to achieve good fluidity in powder form, not to get sticky when having absorbed moisture (thus allowing for easy handling), and to show excellent water absorption and water retention capabilities.
Patent document 2 discloses a water absorbing polymer agent composition. The composition is prepared by sticking inorganic fine powder in a secondary aggregate state to the surface of a water absorbing polymer agent in coarse particle form. This particular method of preparation is a feature of the composition. The composition is intended to achieve excellent fluidity and a high liquid absorption rate.
Patent document 3 discloses modified water absorbing resin particles for use in sanitary products. The particles show an increased absorption rate and causes less gel blocking. The particles are a crosslinked polymer of an unsaturated ethylenic monomer that has an acrylic acid and/or an acrylic acid salt as major structural units. The particles are treated with liquid organic polysiloxane at normal temperature.
Patent document 4 discloses modified water absorbing resin particles. The particles show an increased absorption rate, and cause mitigated moisture-driven blocking and restrained dust production. The particles are prepared by treating water absorbing resin particles with a silicone-based surfactant.
Patent document 5 discloses a water absorbing agent prepared from inorganic powder and water absorbing resin. The agent causes mitigated blocking when having absorbed moisture, offers easy handling, and shows excellent absorption properties under load. The inorganic powder exhibits a pH from 7 to 10, inclusive, when dispersed in a liquid, and a specific surface area of 50 m2/g or more as measured by BET.
Patent document 6 discloses a water absorbent core containing a water absorbing resin and a hydrophilic fiber. The core exhibits an absorbency under load of 10 g/g or less for artificial urine 30 seconds after absorption is started and a water absorbency under load of 20 g/g or more for artificial urine 30 minutes after absorption of water is started. The core is intended to achieve reduced thickness without causing problems in actual use.
Patent document 7 discloses a water absorbent core containing a water absorbing resin and a hydrophilic resin. The water absorbing resin exhibits an absorption swelling pressure of 10,000 Pa or less for physiological saline as the test solution and an absorption swelling pressure of 80,000 Pa or higher 300 seconds after absorption of water is started. The core is intended to achieve excellent liquid diffusibility and a low level of liquid seeping.
Patent document 8 discloses a technique of using 3D spacers in the preparation of a water absorbing resin.
Patent document 9 discloses a particulate water absorbing agent containing water absorbing resin particles and a liquid permeability improver. The particles are prepared by polymerizing (crosslinking) a monomer of an acrylic acid and/or its salt and further crosslinking the surface of the resultant irregularly pulverized particles. The agent is intended to be superior in both physical properties: capillary suction force and liquid permeability.
Patent document 10 discloses a water absorbing resin composition that contains a monomer with carboxyl groups and fumed silica for improved deodorizing effects and fluidity.
Patent document 11 discloses a technique of adding a liquid permeability improver to a water absorbing resin in the preparation of a water absorbing resin.    [Patent Document 1] Japanese Unexamined Patent Publication 56-133028/1981 (Tokukaisho 56-133028; published Oct. 17, 1981)    [Patent Document 2] Japanese Unexamined Patent Publication 64-4653/1989 (Tokukaisho 64-4653; published Jan. 9, 1989)    [Patent Document 3] Japanese Patent 3169133, Specification (registered Mar. 16, 2001)    [Patent Document 4] Japanese Unexamined Patent Publication 9-136966/1997 (Tokukaihei 9-136966; published May 27, 1997)    [Patent Document 5] Japanese Unexamined Patent Publication (Tokukai) 2000-93792 (published Apr. 4, 2000)    [Patent Document 6] Japanese Unexamined Patent Publication (Tokukai) 2003-88551 (published Mar. 25, 2003)    [Patent Document 7] Japanese Unexamined Patent Publication (Tokukai) 2003-88553 (published Mar. 25, 2003)    [Patent Document 8] U.S. Published Patent Application 2002/0128618, Specification (Sep. 12, 2002)    [Patent Document 9] Japanese Unexamined Patent Publication (Tokukai) 2004-261797 (published Sep. 24, 2004)    [Patent Document 10] Published Japanese Translation of PCT Application (Tokuhyo) 2003-500490 (published Jan. 7, 2003)    [Patent Document 11] International Application Published under PCT WO2004/69915 (Aug. 19, 2004)